Document Type: TechNote Legacy Document Number: 8235
Document Type: TechNote
Legacy Document Number: 8235
MicroStation V8 includes the ability for live nesting of reference files in a manner very similar to AutoCAD. Essentially this means that referencing a file causes its children (and potentially their children) to be automatically referenced as well.
MicroStation allows two separate settings that control whether live nesting occurs. The first setting controls whether an attachment is displayed as a nested file. This setting is controlled through the "Ignore Attachment When Live Nesting" checkbox on the attachment settings dialog. AutoCAD has an identical concept except that it refers to attachments of this type (ignored during live nesting) as an "Overlay". Except for the difference in nomenclature there is no difference in this setting or how it is used in the two programs.
The second setting is subtly different than the "Ignore/Overlay" setting discussed above and is supported only by MicroStation. This setting controls whether children of a reference file are displayed at all and if they are displayed, the depth to which the children are nested. The difference is that the parent attachment controls whether the children are displayed rather than the children themselves controlling their nested display. In MicroStation, a nested attachment is displayed only if the child does not have its "Ignore When Live Nesting" setting on AND the parent has live nesting enabled.
The lack of control for reference nesting from the parent attachment can cause some confusion when a set of DGN drawings is converted to AutoCAD format. As AutoCAD interprets every attachment as implicitly displaying its children (unless they are overlays) converting a DGN file to DWG implicitly enables live nesting in the converted file.
At a quick glance it would seem that it would be possible to simulate MicroStation's parental control of nest depth by creating Overlays whenever a parent attachment disables live nesting. Unfortunately this is not possible. Consider the example below.
A MicroStation design file, a.dgn has a single reference, b.dgn that is attached as a standard attachment (not an overlay). In MicroStation it is possible to attach a.dgn as a reference and include the display of b.dgn (as in c.dgn) or to disable the display of children by turning off live nesting (as in d.dgn).
In AutoCAD there is no way to create the set of files above. If reference b.dwg is attached to a.dwg as an overlay then it will not appear in c.dwg. If b.dwg is not attached as an overlay then it will appear in d.dwg when it should not.
The only completely accurate method for resolving this inconsistency is to turn on live nesting (with a high depth value) and to use the "Ignore When Live Nesting" control exclusively to control reference nesting. This usage is enforced when DWG workmode is enabled.
In order to address the somewhat common special case where the no live nesting is used, in version 08.00.02.02 an additional control has been added to the "Advanced" tab of the "Save As" DWG Options dialog. If the "Create Overlays for Reference Attachments" checkbox is selected, then ALL reference attachments are created as overlays - whether or not their "Ignore When Live Nesting" setting is on. This effectively disables live nesting in the DWG output file. This can be useful for a file where live nesting is turned off for all attachments. As Version 7 of MicroStation did not support live nesting, this setting is appropriate for Version 7 files that have not had Live Nesting enabled.